Electronic amplifier circuits are well known in the art. These devices employ various techniques to improve their linearity. There is often a need to maximize output power and efficiency of a linear amplifier circuit by providing sufficient input drive signal levels. However, with any given amplifier there is a drive level beyond which linearity is severly degraded. When overdriven by an input signal most amplifier circuits will inherently generate undesired signals as either adjacent channel frequency splatter or other unwanted signals.
Electronic amplifier circuits that run in Class B or AB are desirable because of their relatively high efficiency and are somewhat linear but require negative feedback to obtain the improved linearity demanded by many applications. Reducing output signal distortion or frequency splatter in a class B or class AB amplifier might permit use of these amplifiers in communications applications requiring relatively distortion-free amplification. Negative feedback reduces distortion as long as the amplifier is not overdriven. Since negative feedback actually increases distortion and unwanted signals when the amplifier is overdriven, it is important to prevent an amplifier from being overdriven.
In many communications applications, adjacent channel frequency splatter caused by an overdriven final amplifier stage in a radio transmitter causes undesirable interference with adjacent channel users. In such applications, which may be typically employed in digital or analog communications systems using signals with a time-varying amplitude, a test signal might be used to determine the maximum allowable input level to the final amplitude stage in order to prevent the final amplifier from being overdriven. In instances where an input test sequence is used to determine the maximum input level to an amplifier, even the input test sequence may cause undesirable adjacent frequency splatter at the instant that the amplifier's maximum drive level has been reached.
As an input signal to an amplifier increases in amplitude, an ideal outptu amplifier circuit would track changes in the input level and amplify the input level producing a substantially exact copy of the input signal. Practical limitations of an electronic amplifier limit the ability of a circuit to infintely follow input signal levels. As an iput signal level increases to the point where the output amplifier stage can no longer amplify it accurately, the output stage will begin to saturate and, if the amplifier's output signal is fed back to the input in a negative feedback loop, an algebraic addition of the amplifier's input signal with its feedback error signal, can produce substantial distortion in the signal input to the amplifier stage.
In some communications applications, it may be desirable to limit the amount of frequency splatter caused by overdriving an amplifier, even during intervals when the maximum input level for the amplifier is being determined by a test sequence. An amplifier circuit that includes the ability to limit adjacent frequency splatter even when a maximum drive level is being measured by a test sequence would be an improvement over the prior art.